This program of research is exploring the possible role of the diet in influencing brain functions thought to be controlled at least in part by serotonin-containing neurons in the brain. It is also identifying effects of the diet on other, chemical parameters of the brain, including amino acids and other amine transmitters. The work now in progress includes studies on (1) the effect of tryptophan or neutral amino acid administration, or of the ingestion of particular foods, on brain serotonin and growth hormone secretion; (2) the effect of single meals on the brain levels of each of the acidic, basic, and neutral amino acids; (3) the effects of amino acid and diet-induced changes in brain serotonin on sensitivity to painful stimuli; (4) the importance of indoleamine-2,3-dioxygenase in the normal metabolism of serotonin in gut and brain. The results should help to illuminate possible functional consequences to the body of having a particular subset of brain neurons whose transmitter levels are coupled to peripheral metabolic state and diet.